1. Technical Field
The embodiments described herein relate to a circuit for a front-end tunable filter of a communication and broadcast receiver and a tuning method thereof.
2. Related Art
The receiver has been widely used in the field of mobile phones, television, broadcast, global positioning system (GPS). For example, radio broadcasting and television signals are distributed in different frequency bands in China. For each corresponding frequency band, each program has its own channel. For example, the FM broadcast RF signals are from 88 MHz to 108 MHz. Each FM channel is 200 KHz.
Table of China's Radio and TV Frequency Allocations
WaveChannelBandFrequency (MHz)IntervalUseLF (LW)120-300 KHz —Long wave AMbroadcastingMF (AM)525 KHz-1605 KHz    9 KHzMedium wave AMbroadcastingHF (SW) 3.5-29.7 MHz9 KHzShort wave AMradio andsingle-side bandcommunicationVHF (FM) 88-108 MHz200 KHz FM broadcastingand databroadcastingVHF48.5-92 MHz 8 MHzTV and databroadcastingVHF167-223 MHz8 MHzTV and databroadcastingUHF223-443 MHz8 MHzTV and databroadcastingUHF443-870 MHz8 MHzTV and databroadcasting
The receiver requires the RF signal received by the RF front-end to be transferred to the baseband through one or more stages of frequency conversion for the demodulation processing. For example, in the topology of the two-stage frequency conversion, the received RF signal is first converted to an intermediate frequency for amplification and filtering, and then converted to a baseband for demodulation processing. In the FM broadcasting, for example, the 10.7 MHz is often used as the receiver IF in the FM receiver. First, the RF front-end of the receiver receives a FM broadcast signal. Next, the received signal is converted to 10.7 MHz for filtering; the obtained IF signal can be amplified if necessary. Then, the 10.7 MHz FM signal is further converted to a baseband for demodulation processing by a demodulation circuit. Finally the broadcast program signal is obtained. Other common receiver topologies also include: a zero IF topology, which directly converts the received RF signal to a baseband for demodulation processing without the IF stage; as well as a multi-IF topology, which ultimately converts the signal to a baseband through multi-stage IF conversion for demodulation processing. The working principles of the other receivers are the same.
FIG. 1 illustrates a receiver system 100. The receiver system includes an RF front end 101, a frequency conversion unit 107, a local oscillation circuit (LO) 110, and a baseband demodulation circuit 114. The RF front end 101 receives a signal 130, a RF signal 134 is obtained after processing; the processing of the input signal 130 by the RF front end 101 may be either a gain processing including amplification or attenuation; or a filtering processing including filtering or attenuation of out of band signal to retain useful signal in the band. The local oscillation circuit 110 generates a tuned local oscillator signal 132 with a frequency of FLO. The frequency conversion unit 107 receives the processed RF signal 134 and the local oscillation signal 132, to obtain a baseband signal 135 with a frequency FIF. The demodulation circuit 114 demodulates the baseband signal 135 to generate a desired demodulated signal 138.
FIG. 2 illustrates another receiver system 110. The receiver system 110 further includes a pre-filter 104. The pre-filter 104 can selectively filter the input signal 130 according to a pre-determined channel, thus filtering or attenuating the out of band signal. Then the resultant inband signal 131 is input to the RF front end.
FIG. 3 illustrates another receiver system 111. The receiver system 111 further includes an antenna 102. The antenna receives a wireless RF signal in the air to be filtered. If the antenna is an active antenna, the antenna can have the function of signal amplification. The RF signal 130 is then transmitted to the pre-filter 104. Here, the antenna of the radio receiver can be regarded as a pre-filter, or a part of the pre-filter.
FIG. 4 illustrates yet another receiver system 112. The receiver system 112 further includes a tuning control loop for the pre-filter 104 to help the receiver to tune to the received programming channel. For this receiver system, when the pre-filter 104 is tuning, in addition to the calibration feedback circuit 116 and the pre-filter 104, the frequency conversion unit 107 and the baseband demodulation circuit 114 module in the signal reception link must also work. The baseband demodulation circuit 114 also needs to generate a feedback signal 126 to the calibration feedback circuit 116. This feedback signal 126 is used to indicate the strength of the RF signal 130 and the frequency deviation. The calibration feedback circuit 116 generates a tuning calibration signal 120 and a tuning control signal 122 according to the feedback signal 126. The tuning control signal 122 controls the switch 118. When the pre-filter is tuning, the switch 118 is on, and the tuning calibration signal 120 is sent to the RF signal 130. The calibration feedback circuit 116 also generates a control signal 124 for the pre-filter 104. The control signal 124 is used to adjust the tunable component value of the pre-filter 104. The pre-filter 104 has different frequency response to the tuning calibration signal 120 with changes in the control signal 124, reflecting in the RF signal 130. As a result, the RF signal 130 reaches the baseband demodulation circuit 114 along the signal reception link. With the calibration feedback circuit 116 and the demodulation circuit 114 continuously adjusting the signals 120, 122, 124 of the calibration feedback circuit, the front filter 104 is ultimately tuned to the optimum frequency desired by the receiver system.
The tuning scheme of the system 112 has the following disadvantages. First, the tuning scheme of the system 112 is dependent on the normal working of the signal reception link. Secondly, the tuning calibration signal 120 generated by the calibration feedback circuit 116 must have a frequency difference of an intermediate frequency FIF with the local oscillator signal 132. Only in this way can the baseband signal 135 generated by the frequency conversion unit 107 be exactly in the intermediate frequency FIF of the signal reception link in the tuning process. This circuit implementation of this tuning scheme needs an oscillator included in the calibration feedback circuit 116, whose operating frequency is different from the operating frequency of the local oscillator 132 FLO by a frequency FIF. Third, this tuning scheme needs separate processing of the baseband signal 135 in the tuning process by the base-band demodulation circuit 114 to generate the desired feedback control signal 126. This portion of the function is an extra to the demodulation function, increasing the cost of the circuit. Fourth, the control loop of the tuning scheme is lengthy and involved a plurality of modules in the signal reception link. As a result, the calibration feedback circuit 116 is subject to many restrictions and is extremely difficult to optimize. Fifth, in this tuning scheme, the design of the calibration feedback circuit 116 is heavily dependent on the structure of the signal reception link resulting in the lack of the independence, reusability, and portability of the calibration feedback circuit 116.